Fuel cell stack with enhanced freeze-thaw durability

ABSTRACT

The present invention provides a fuel cell stack with enhanced freeze-thaw durability. In particular, the fuel cell stack includes a gas diffusion layer between a membrane-electrode assembly and a bipolar plate. The gas diffusion layer has a structure that reduces contact resistance in a fuel cell and is cut at a certain angle such that the machine direction (high stiffness direction) of GDL roll is not in parallel with the major flow field direction of the bipolar plate, resulting in an increased GDL stiffness in a width direction perpendicular to a major flow field direction of a bipolar plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/078,168 filed in the United States Patent and Trademark Office onApr. 1, 2011, which claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Application No. 10-2010-0122439 filed Dec. 3, 2010, the entirecontents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a fuel cell stack with enhancedfreeze-thaw durability. More particularly, it relates to a fuel cellstack that is designed to reduce contact resistance in a fuel cell toprevent water that is a by-product of an electrochemical reaction frombeing frozen under a sub-zero temperature condition when reactant gasessuch as hydrogen and oxygen gases are supplied to the fuel cell stack.

(b) Background Art

Polymer electrolyte membrane fuel cells (PEMFCs) have been widely usedas a fuel cell for a vehicle. For a fuel cell stack, manufactured bystacking hundreds of unit cells of the polymer electrolyte membrane fuelcell, to be properly mounted in a vehicle, it is essential that it showshigh power performance of at least tens of kilowatts (kW), and thusrequires stable operation in a wide range of current density.

In a reaction for generating electricity in a fuel cell, after hydrogensupplied to the anode at which oxidation occurs in a membrane-electrodeassembly (MEA) of the fuel cell is divided into hydrogen ions (protons)and electrons, hydrogen ions move to the cathode at which reductionoccurs through a polymer electrolyte membrane, and electrons move to thecathode through an external circuit. Also, in the cathode, oxygenmolecules, hydrogen ions, and electrons react with each other togenerate electricity and heat and water as a by-product.

If a suitable amount of water is generated from the electrochemicalreaction in the fuel cell, the generated water may serve to maintainsuitable humidity conditions for the membrane-electrode assembly.However, if the amount of water generated is excessive, the excessivewater may not be removed at a high current density, thus causingflooding of water throughout the cell. The flooding may prohibitreactant gases from being efficiently supplied to the fuel cell, thusdeepening a voltage loss.

Water generates from the reaction between hydrogen and oxygen in the airin the polymer electrolyte membrane fuel cell. If the freeze-thaw cycleis repetitively changed from a sub-zero temperature to an ordinarytemperature, components of the fuel cell and interfaces between thecomponents such as an MEA and a gas diffusion layer (GDL) may bephysically damaged thereby reducing its electrochemical performance anddurability. Therefore, for the stable operation of a hydrogen fuel cellvehicle, it is crucial to increase the durability of a fuel cell stackunder such a freeze-thaw cycle condition.

Various attempts have been conducted to increase the freeze-thawdurability of a typical fuel cell. For example, Korean Pat. No.10-0802749, registered in 2008, discloses a technology of increasing thedurability by optimizing a fuel cell cooling line structure to reducethe freeze-thaw cycle. U.S. Pat. Application Publication Nos.2010/0143813 and 2008/0102326 disclose technologies of increasing freezestart capability by optimizing a method for controlling operation of afuel cell. Also, U.S. Pat. Application Publication No. 2008/0241608discloses a method of operating a fuel cell by removing ice generated ata sub-zero temperature by heat. However, these methods are too complexto apply in reality, and their effects are also limited. Accordingly,for a mass production of hydrogen fuel cell vehicles, it is necessary todevelop a new technology to improve the freeze-thaw durability while atthe same time making the implementation process as simple as possible.

As commercialization of fuel cells progresses, much research anddevelopment is being conducted on a gas diffusion layer (GDL) that is anessential component for managing water in a fuel cell. A GDL is attachedto the outer surface of anode and cathode catalyst layers in an MEA of afuel cell to perform various functions such as supply of reactant gases(hydrogen and oxygen gases in the air), transport of electrons generatedfrom an electrochemical reaction, and minimize flooding in the fuel cellby discharging water generated from the reaction.

A GDL has been currently commercialized has a dual layer structure of amicroporous layer (MPL) and a macro-porous substrate (or backing). TheMPL has a pore size of less than about 1 μm when measured by a mercuryintrusion method. The macro-porous substrate, on the other hand, has apore size of about 1 μm to about 300 μm.

The MPL of the GDL may be manufactured by mixing carbon power such asacetylene black carbon and black pearls carbon with hydrophobic agentbased on polytetrafluoroethylene (PTFE) and fluorinated ethylenepropylene (PEP), and then may be coated on one or both surfaces of themacro-porous substrate according to applications. On the other hand, themacro-porous substrate of the GDL may be typically formed of carbonfiber and hydrophobic agents based on PTFE or PEP, and may includecarbon fiber cloth, carbon fiber felt, and carbon fiber paper.

Since the GDL for the fuel cell has to be designed to have appropriateperformance according to operation conditions and specific applicationfields of the fuel cell for, e.g., transportation, portable, andresidential power generation, the GDL based on either carbon fiber feltor carbon fiber paper (in which overall characteristics such as supplyof reactant gas, discharge of generated water, andcompressibility/handling property for stack assembly are excellent), ismore widely used for a fuel cell vehicles than carbon fiber cloth.

Also, a GDL has a significant influence on performance of a fuel cellaccording to various characteristics such as gas permeability,compressibility, degree of hydrophobicity of MPL and macro-poroussubstrate, structure of carbon fiber, porosity/pore distribution,tortuosity of pore, electrical resistance, and bending stiffness.Particularly, the GDL has a significant influence on the performance inthe mass transport zone.

The gas diffusion layer needs to show excellent performance in a fuelcell, and have appropriate stiffness for excellent handling propertieswhen hundreds of unit cells are assembled into a fuel cell stack. On theother hand, when the stiffness of the gas diffusion layer is too high ina direction of a roll, the gas diffusion layer is difficult to store ina roll form, thus reducing its mass-productivity capabilities.

Alternatively as noted above, when the stiffness of a gas diffusionlayer 106 is deficient in a fuel cell, as shown in FIG. 1, the GDL 106may intrude into a flow field channel 202 of a bipolar plate (alsocalled as a separator) 200 (thus causing GDL intrusion). Thus, when theGDL 106 intrudes into the flow field channel 202 of the bipolar plate200, a channel space for transferring materials such as reactant gasesand generated water may not have enough room. Also, since the contactresistance between the GDL 106 and the rib (or land) 204 of the bipolarplate 200 and between the GDL 106 and an MEA 100 increases, theperformance of the fuel cell may be considerably reduced.

Particularly, when the contact resistance in a cell increases, aninterface between the GDL and the MEA or between the GDL and the bipolarplate may not be suitably maintained to generate an unnecessary gap. Inthis case, water generated in the fuel cell may be frozen to ice in theunnecessary gap under a freeze-thaw condition.

Thus, when there is ice generation, repetitive freeze-thaw cycles maydamage the interface between components in the fuel cell. Accordingly,in order to increase the durability of a fuel cell, it is important toreduce the contact resistance so as not to generate a gap at theinterface among the components of the fuel cell.

Generally, a bipolar plate for a fuel cell includes a major flow fieldand a minor flow field. Here, it is necessary for a GDL not to intrudeinto a channel of the major flow field. Therefore, it is important toincrease the stiffness of the GDL oriented in the width (W) directionwhich is perpendicular to the major flow field direction of the bipolarplate than that oriented in the length (L) direction which is parallelwith the major flow field direction of the bipolar plate (see FIGS. 2and 3). Otherwise, as shown in FIG. 1, when a GDL having a low stiffnessis arranged in the width direction of the major flow field of thebipolar plate, the GDL may further intrude into the major flow fieldchannel of the bipolar plate. Accordingly, since a space in which icemay be generated at a sub-zero temperature (due to increase of damage ordeformation of the interface in the fuel cell) increases, thefreeze-thaw durability of the fuel cell may be reduced.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

The present invention provides a fuel cell stack that can reduce contactresistance in a fuel cell and improve freeze-thaw durability, using aGDL that is manufactured by using a typical art stack fabricationprocess without an additional modification of the process and optimizinga method of cutting the GDL in a sheet size appropriate to the fuelcell, i.e., a GDL in which its stiffness in a width directionperpendicular to a major flow field direction of a bipolar plate isincreased by cutting a rolled GDL material at a certain angle such thata machine direction (MD) or high stiffness direction of GDL roll is notin parallel with the major flow field direction of the bipolar plate inorder to minimize the intrusion of the GDL into a gas channel of thebipolar plate.

In one aspect, the present invention provides a fuel cell stack withenhanced freeze-thaw durability, the fuel cell stack including a GDLbetween an MEA and a bipolar plate, wherein the GDL has a structure thatreduces contact resistance in a fuel cell and the GDL has a stiffness ina width direction perpendicular to a major flow field direction of abipolar plate that is increased by cutting a rolled GDL material at acertain angle such that the MD (high stiffness direction) of GDL roll isnot in parallel with the major flow field direction of the bipolarplate.

In some embodiments of the present invention, the GDL may be cut suchthat an angle between the MD (high stiffness direction) of the GDL rolland the major flow field direction of the bipolar plate is greater thanabout 0 degree, and equal to or smaller than about 90 degrees.

In another embodiment, the GDL may be cut such that an angle between theMD (high stiffness direction) of the GDL roll and the major flow fielddirection of the bipolar plate is greater than about 25 degrees, andequal to or smaller than about 90 degrees. In still another embodiment,the GDL may have a Taber bending stiffness of the MD (high stiffnessdirection) of the GDL roll that ranges from about 20 g_(f)·cm to about150 g_(f)·cm.

In yet another embodiment, the GDL may have a Taber bending stiffness ofthe MD (high stiffness direction) of the GDL roll that ranges from about50 g_(f)·cm to about 100 g_(f)·cm.

In still yet another embodiment, the GDL may include a MPL contacting anouter surface of each electrode of an MEA, and a macro-porous substratecontacting a flow field of the bipolar plate, and the macro-poroussubstrate may be formed of one of carbon fiber felt and carbon fiberpaper or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a diagram illustrating intrusion of a GDL into a main flowfield of a bipolar plate by bipolar plate land compression when fuelcells are assembled to each other in a typical stack;

FIG. 2 is a diagram illustrating a method (90° GDL) of cutting the sheetof a GDL according to an exemplary embodiment of the present invention,compared to a method (0° GDL) of cutting the sheet of a GDL according toa related art;

FIG. 3 is a diagram illustrating (a) an arrangement of a GDL's MD (highstiffness direction) and a major flow field direction of the bipolarplate in a 0° GDL-applied stack according to related art and (b) anarrangement of a GDL's MD (high stiffness direction) and a major flowfield direction of the bipolar plate in a 90° GDL-applied stackaccording to an exemplary embodiment of the present invention;

FIG. 4 is a scanning electron microscope (SEM) photograph (500×)illustrating a macro-porous substrate of the GDL used in an exemplaryembodiment of the present invention and in the related art;

FIG. 5 is a graph illustrating the electrochemical performances beforeand after 1000 freeze-thaw cycles of the 0° GDL-applied stack accordingto a related art and the 90° GDL-applied stack according to an exemplaryembodiment of the present invention;

FIG. 6 is graphs illustrating the cell voltage decay as a function ofnumber of freeze-thaw cycles of the 0° GDL-applied stack according to arelated art and the 90° GDL-applied stack according to an exemplaryembodiment of the present invention, in which graph (a) shows a resultat a current density of 800 mA/cm² and graph (b) shows a result at acurrent density of 1,400 mA/cm²;

FIG. 7 is a graph illustrating the high frequency resistances before andafter 1000 freeze-thaw cycles of the 0° GDL-applied stack according to arelated art and the 90° GDL-applied stack according to an exemplaryembodiment of the present invention;

FIG. 8 is graphs illustrating the increase of high frequency resistanceas a function of number of freeze-thaw cycles of the 0° GDL-appliedstack according to a related art and the 90° GDL-applied stack accordingto an exemplary embodiment of the present invention, in which graph (a)shows a result at a current density of 800 mA/cm² and graph (b) shows aresult at a current density of 1,400 mA/cm².

Reference numerals set forth in the Drawings includes reference to thefollowing elements as further discussed below:

100: MEA 106: GDL 200: bipolar plate 202: bipolar plate channel 204:bipolar plate land

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

Hereinafter, exemplary embodiment of the present invention will bedescribed in detail with reference to the accompanying drawings.

The present invention provides a fuel cell stack with enhancedfreeze-thaw durability by reducing generation of ice in a fuel cellthrough application of a GDL that can minimize contact resistance in thecell under a fuel cell freeze-thaw cycle condition.

The inherent anisotropic characteristics of a gas diffusion layer may beused to reduce the contact resistance in a fuel cell.

In a conventional manufacturing process, in a GDL including carbon fiberfelt or carbon fiber paper that is widely used for a fuel cell vehicleas a macro-porous substrate, the carbon fiber may be preferentiallyoriented in one direction (i.e., MD) to have mechanical properties suchas bending stiffness and tensile stress greater than those in the otherdirection (i.e., cross-machine direction (CMD), also called astransverse direction (TD)). Accordingly, the MD of fabrics of a GDL rollmay usually be a high stiffness direction, and the CMD may be a lowstiffness direction.

In the fuel cell stack according to the exemplary embodiment of thepresent invention, an angular range between the MD (high stiffnessdirection) of the GDL roll and the major flow field direction of thebipolar plate may be greater than about 0 degrees, and equal to orsmaller than about 90 degrees. More preferably, the GDL roll may be cutsuch that the angular range becomes greater than about 25 degrees, andequal to or smaller than about 90 degrees.

More specifically, as shown in a related art of FIG. 3A, when a GDL rollis cut, it may be cut such that the MD of the GDL runs parallel to themajor flow field direction of the bipolar plate. However, as shown in anembodiment of FIG. 3B, a GDL may be cut such that the MD of the GDL doesnot run parallel to the major flow field direction of the bipolar plate.Accordingly, the stiffness of the GDL may increase in the width (W)direction that crosses the major flow field direction of the bipolarplate.

It has been described as an example that the cutting of the GDL roll isperformed such that the MD of GDL roll and the major flow fielddirection of the bipolar plate cross each other at an angle equal to orsmaller than about 90 degrees. For example, the GDL roll may be cut suchthat the two directions cross each other at an angle of about 30degrees, 45 degrees, and 60 degrees in order to increase the freeze-thawdurability of the fuel cell stack.

Thus, the GDL intrusion into gas channels of the bipolar plate can bereduced and an unnecessary gap in which water is frozen into ice can bereduced at an interface between the GDL and the MEA or the GDL and thebipolar plate. Accordingly, the freeze-thaw durability of the fuel cellstack can be improved.

The Taber bending stiffness of the MD (high stiffness direction) of theGDL roll may range from about 20 g_(f)·cm to about 150 g_(f)·cm, and insome instances from about 50 g_(f)·cm to about 100 g_(f)·cm. If theTaber bending stiffness is smaller than about 20 g_(f)·cm, the stiffnessis too small for the GDL to be used for a fuel cell vehicle. If theTaber bending stiffness is greater than 150 g_(f)·cm, the GDL becomes sostiff that the GDL cannot be stored in a roll form, thus causing areduction in the mass-productivity of GDL.

Also, the macro-porous substrate of the GDL mounted in the stackaccording to an exemplary embodiment of the present invention may beconfigured with carbon fiber felt, carbon fiber paper, or a combinationthereof. Thus, since the MD (high stiffness direction) of the GDL andthe major flow field direction of the bipolar plate are not parallel toeach other, and carbon fiber felt or carbon fiber paper is used as amacro-porous substrate of the GDL, the contact resistance of the fuelcell can be reduced, and the interface between components of the fuelcell can be suitably maintained, thereby minimizing ice generation.

That is, the decrease in performance of the fuel cell due to increase inthe contact resistance can be reduced by decreasing the contactresistance between the GDL and the bipolar plate land, or between theGDL and the MEA. Also, since the interface between the GDL and MEA, orbetween the GDL and the bipolar plate can be suitably maintained, a gapin which generated water is frozen into ice may be significantlyreduced, thereby improving the freeze-thaw durability.

The fundamental characteristics of the carbon fiber felt-type GDL usedin the present embodiment will be described in Table 1 below. Themacro-porous substrate may be configured with typical carbon fiber feltas shown in FIG. 4 that is a magnified view of about 500× by a scanningelectron microscope. It can be seen that the carbon fibers areirregularly entangled.

As described in Table 1 below, the bending stiffness of the GDL has beenmeasured with respect to MD and CMD at a bending angle of 15 degreesusing a Taber Industries Stiffness Tester.

TABLE 1 Type of Macro- Weight Bending Stiffness Porous Thickness perUnit [Taber Stiffness Unit, g_(f) · cm] Substrate [μm] Area [gm⁻²] MDCMD Carbon Fiber 426 ± 10 135 ± 2 64.87 ± 6.97 12.62 ± 0.38 Felt

Hereinafter, an embodiment and a test example of the present inventionwill be described in further detail.

Embodiment

As an embodiment of the present, a GDL roll was cut such that the MD(high stiffness direction) of the GDL roll is perpendicular to the majorflow field direction of the bipolar plate (cutting angle of about 90degrees). The GDLs were assembled into a 5-cell stack together withoverall components such as MEAs, metallic bipolar plates, end plates andother assembly members.

Comparative Example

As a comparative example, a GDL roll was cut such that the MD (highstiffness direction) of the roll runs parallel to the major flow fielddirection of the bipolar plate (cutting angle of about 0 degree). TheGDLs were assembled into a 5-cell stack together with overall componentssuch as MEAs, metallic bipolar plates, end plates and other assemblymembers.

Test Example

Electrochemical performance of the GDLs according to the embodiment andthe comparative example was tested. That is, the electrochemicalperformance of fuel cell stacks including the GDLs according to theexemplary embodiment and the comparative example was compared bymeasuring the current-voltage (I-V) polarization characteristics basedon a 5-cell stack. A typical commercialized tester was used as a testerfor measuring electrochemical cell performance.

In this case, the test of the electrochemical performance of the fuelcell stacks having the GDLs according to the illustrative embodiment ofthe present invention and the comparative example were performed underthe following conditions.

-   -   Temperature at the inlet of the fuel cell=65° C.,    -   Hydrogen anode/air cathode relative humidity (RH)=50%/50%,    -   Hydrogen anode/air cathode stoichiometric ratio (S.R.)=1.5/2.0

A freeze-thaw cycle condition applied to the exemplary embodiment of thepresent invention and the comparative example includes putting afive-cell stack into an environmental chamber in which the temperatureis adjustable, repeating 1,000 cycles at chamber temperatures between−25° C. and 15° C., and measuring and comparing the electrochemicalperformance and high frequency resistance (HFR) of a stack for every 50cycles. Here, the HFR that was measured may be a factor representingcontact resistance in the cell. If the HFR increases, an interfacebetween components is damaged or deformed, and contact becomes poor. Inthis instance, the HFR was measured by a typical commercialized testerunder a condition of amplitude of about 5 A and frequency of about 1kHz.

Test results of the electrochemical performance of the fuel cell stacksincluding the GDLs according to the illustrative embodiment of thepresent invention and the comparative example are shown in FIGS. 5through 8.

The electrochemical performance of the stacks having the GDLs accordingto the present embodiment (90° GDL) and the related art (0° GDL) wascompared with each other after completion of 1,000 freeze-thaw cycles.As shown in FIG. 5, the electrochemical performance of the stacks of theembodiment and the comparative example were both reduced. On thecontrary, the stack including the GDL according to the presentembodiment showed a greater increase in electrochemical performance thanthe stack including the GDL according to the related art after both 0freeze-thaw cycles and 1,000 freeze-thaw cycles. Further, the same wastrue for the performance decay rate.

For the quantitative evaluation of the electrochemical performance decayrate of the stacks as a function of the number of the freeze-thawcycles, a medium current density of about 800 mA/cm² and a high currentdensity of about 1,400 mA/cm² were selected as representative fuel celloperation conditions. Cell voltage drops at the above current densitieswere compared to each other. As shown in FIG. 6A, when the currentdensity was 800 mA/cm², the cell voltage of the stack including the GDLaccording to the related art (0° GDL) decreased at a rate of about −38μV/cycle, but the cell voltage of the stack including the GDL accordingto the present embodiment (90° GDL) decreased at a rate of about −27μV/cycle. Accordingly, it can be seen that the performance of the cellstack according to the present invention was more gradually decreasedthan that of the cell stack according to the related art.

As shown in FIG. 6B, when the current density was 1,400 mA/cm², the cellperformance decay rate increased. The cell voltage of the stackincluding the GDL according to the related art decreased at a rate ofabout −109 μV/cycle, but the cell voltage of the stack including the GDLaccording to the present embodiment decreased at a rate of about −66μV/cycle. Accordingly, it can be seen that the performance of the cellstack according to the present invention was more gradually decreased atthe high current density as well.

When changes in the contact resistance in the cell of the fuel cellstack were compared to each other, the HFRs with respect to the stackaccording to the comparative example of the related art and the stackaccording to the present embodiment were compared to one another after1,000 freeze-thaw cycles were completed. As shown in FIG. 7, the HFRsincreased in both the stack according to the present invention and thestack according to the related art after 1,000 freeze-thaw cycles.

However, the HFR with respect to the stack according to the presentembodiment was smaller than the stack according to the stack accordingto the related art after both 0 cycles and 1,000 cycles. This means thatthe contact state between components in the stack cell of the presentembodiment is greater than that in the stack cell according to therelated art. Accordingly, it is very unlikely that ice may be generatedat interfaces between components in the stack according to the presentembodiment at a sub-zero temperature, and thus the cells may be lessdamaged under freezing conditions.

Additionally, the HFR increase rates as a function of the number of thefreeze-thaw cycles were quantitatively measured. As shown in FIG. 8A,when the current density was 800 mA/cm², the HFR of the stack accordingto the related art increased at a rate of about 43 μΩcm²/cycle, but theHFR of the stack according to the present embodiment increased at a rateof 34 μΩcm²/cycle. Accordingly, it can be seen that the HFR of the stackaccording to the present embodiment increased more gradually than thatof the stack according to the related art.

As shown in FIG. 8B, when the current density is about 1,400 mA/cm², theHFR increase rate increases in both stacks. The HFR of the stackaccording to the related art increased at a rate of about 50μΩcm²/cycle, but the HFR of the stack according to the present inventionincreased at a rate of about 38 μΩcm²/cycle. Accordingly, it can be seenthat the HFR of the stack according to the present embodiment increasedmore gradually at the high current density.

For reference, the decay rate of the cell voltage and the increase rateof the HFR after 1,000 freeze-thaw cycles are summarized as shown inTable 2 below.

TABLE 2 Cell Voltage Decay Rate HFR Increase Rate [μV/cycle] [μΩcm²/cycle] 800 1,400 800 1,400 Type of Stack mAcm⁻² mAcm⁻² mAcm⁻² mAcm⁻²0° GDL Stack −38 −109 43 50 according to Related Art 90° GDL Stack −27−66 34 38 according to Present Invention

As described above, compared to the stack including the GDL (the MD ofthe GDL roll is parallel to the major flow field direction of thebipolar plate (cutting angle is 0 degree)) according to the related art,the stack including the GDL (the MD of the GDL roll is perpendicular tothe major flow field direction of the bipolar plate (cutting angle is 90degrees) according to the present embodiment has higher electrochemicalperformance, slower performance decay rate, smaller contact resistancein the cell, and a slower contact resistance increase rate during thefreeze-thaw cycle. Accordingly, since the probability that ice isgenerated at interfaces in the cell is low, the freeze-thaw durabilitycan be improved.

According to an embodiment of the present invention, the fuel cell stackwith the GDL in which its stiffness in a width direction perpendicularto a major flow field direction of a bipolar plate is increased bycutting a rolled GDL material at a certain angle such that a MD (highstiffness direction) of GDL roll is not in parallel with the major flowfield direction of the bipolar plate can have enhanced durability underfreeze/thaw cycling conditions. Also, the freeze-thaw durability may beimproved by reducing generation of ice at interfaces in the fuel cell.

The invention has been described in detail with reference to embodimentsthereof. However, it will be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe appended claims and their equivalents.

What is claimed is:
 1. A method of manufacturing a fuel cell stack withenhanced freeze-thaw durability, the fuel cell stack comprising aplurality of fuel cells, each of the fuel cells comprising a polymerelectrolyte membrane, catalyst layers, compressible gas diffusion layersand bipolar plates, wherein the compressible gas diffusion layer (GDL)is attached to an outer surface of each of catalyst layers coated onboth sides of the polymer electrolyte membrane, the bipolar plate isattached to an outer surface of each of the gas diffusion layers and iscomposed of a major flow field and a minor flow field, and the gasdiffusion layer has a width direction perpendicular to a major flowfield direction of the bipolar plate and a length direction which is inparallel with the major flow field direction of the bipolar plate, themethod comprising: providing a rolled compressible GDL material having adual layer structure including a microporous layer and a macroporoussubstrate which is formed of carbon fiber felt, or carbon fiber paper,wherein a machine direction of the rolled compressible GDL material isdirected to an inherent high stiffness direction of the compressible GDLand a cross-machine direction thereof is directed to a low stiffnessdirection of the compressible GDL, wherein the machine direction has ahigher stiffness than the cross machine direction, cutting the rolledcompressible GDL material to form the compressible GDL at a certainangle (θ), wherein during the cutting, the certain angle is formedbetween the machine direction of the compressible GDL material and themajor flow field direction of the bipolar plate such that the inherenthigh stiffness direction of the compressible GDL is not in parallel withthe length direction of the compressible GDL, and assembling thecompressible GDL with the bipolar plate such that the certain angle isformed between the inherent high stiffness direction of the compressibleGDL and the major flow field direction of the bipolar plate.
 2. Themethod of claim 1, wherein the compressible GDL is cut from the rolledcompressible GDL material at the certain angle of 60°≦θ≦90° such that anangle between the inherent high stiffness direction of the compressibleGDL and the major flow field direction of the bipolar plate is in arange of 60°≦θ≦90°.
 3. The method of claim 1, wherein the compressibleGDL is cut from the rolled compressible GDL material at the certainangle of 90° such that an angle between the inherent high stiffnessdirection of the compressible GDL and the major flow field direction ofthe bipolar plate is 90 degrees.
 4. The method of claim 1, wherein thecompressible GDL has a Taber bending stiffness of the machine direction(high stiffness direction) of the gas diffusion layer roll that rangesfrom 20 gf·cm to 150 gf·cm.
 5. The method of claim 1, wherein thecompressible GDL has a Taber bending stiffness of the machine direction(high stiffness direction) of the gas diffusion layer roll that rangesfrom 50 gf·cm to 100 gf·cm.
 6. A method of manufacturing a compressiblegas diffusion layer (GDL) for a fuel cell with enhanced freeze-thawdurability, the method comprising: providing a rolled compressible GDLmaterial having a dual layer structure including a microporous layer anda macroporous substrate which is formed of carbon fiber felt, or carbonfiber paper, wherein a machine direction of the rolled compressible GDLmaterial is directed to an inherent high stiffness direction of thecompressible GDL and a cross-machine direction thereof is directed to alow stiffness direction of the compressible GDL, wherein the machinedirection has a higher stiffness than the cross machine direction,cutting the rolled compressible GDL material to form the compressibleGDL at a certain angle, wherein during the cutting, the certain angle isformed between the machine direction of the compressible GDL materialand a major flow field direction of the bipolar plate, such that theinherent high stiffness direction of the compressible GDL is arranged inone direction and the inherent high stiffness direction of thecompressible GDL is not parallel with the length direction of thecompressible GDL, wherein the certain angle (θ) is 60°≦θ<90° degrees. 7.A method of manufacturing a fuel cell stack with enhanced freeze-thawdurability, the fuel cell stack comprising a plurality of fuel cells,each of the fuel cells comprising a polymer electrolyte membrane,catalyst layers, compressible gas diffusion layers and bipolar plates,wherein the compressible gas diffusion layer (GDL) is attached to anouter surface of each of catalyst layers coated on both sides of thepolymer electrolyte membrane, the bipolar plate is attached to an outersurface of each of the gas diffusion layers and is composed of a majorflow field and a minor flow field, and the gas diffusion layer has awidth direction perpendicular to a major flow field direction of thebipolar plate and a length direction which is in parallel with the majorflow field direction of the bipolar plate, the method comprising,cutting a rolled compressible GDL material to form the compressible GDLat an angle in a range of 60°≦θ<90° with respect to the major flow fielddirection of the bipolar plate, wherein the machine direction of therolled compressible GDL material is directed to an inherent stiffnessdirection of the formed compressible GDL, and assembling thecompressible GDL with the bipolar plate such that the angle is formedbetween the inherent stiffness direction of the compressible GDL and themajor flow field direction of the bipolar plate and the inherentstiffness direction of the compressible GDL is at the angle 60°≦θ<90°with respect to the length direction of the compressible GDL, and at asame time, with respect to the major flow field direction of the bipolarplate in the fuel cell stack, to thereby reduce contact resistance atinterfaces in a fuel cell and increase a stiffness in a width directionperpendicular to the major flow field direction of the bipolar plate.